Inverse latexes based on polyelectrolytes, including partially or completely salified 2-methyl-2-[(1-oxo-2-propenyl)amino]-1-propanesulfonic acid (also known as 2-acrylamido-2-methylpropanesulfonic acid, ATBS or AMPS), and their use in cosmetics and/or pharmaceuticals have formed the subject of numerous patent applications. However, the presence of large amounts of water and oil in these inverse latexes represents a not insignificant disadvantage in terms of volume, cost and sometimes increased risks and/or toxic effects.
Solutions have thus been developed for increasing the concentration of polyelectrolytes in the final inverse latexes, for example by subjecting the reaction medium, at the end of polymerization, to a vacuum distillation stage in order to remove a more or less large portion of water and oil. However, this distillation is problematic to carry out as it often brings about destabilization of the inverse latex, which has to be controlled by the prior addition of stabilizing agents.
European patent applications EP 0 161 038 and EP 0 126 528, and also British patent application GB 1 482 515, disclose such a use of stabilizing polymers. The disadvantage of these stabilizing polymers is that they comprise alcohols or glycols, which can cause environmental problems. Furthermore, the reaction medium sometimes sets solid during the distillation stage, without this phenomenon ever having been truly explained, but the certain consequence of which is the destruction of the batch of inverse latex in the course of manufacture and tiresome and expensive cleaning of the reactor used. Finally, even when the distillation takes place correctly, the inverse latexes obtained are often inverted with difficulty during the processing thereof in an aqueous phase. They also exhibit a high viscosity and sometimes exhibit microgels within them. These disadvantages thus prevent them from being used in the manufacture of cosmetic formulations. In order to overcome these disadvantages, the inventors have developed an inverse latex disclosed in the French patent application published under the number FR 2 879 607, comprising 50% to 80% by weight of a poly-electrolyte comprising from 0.01 mol % to 10 mol % of at least one monomer unit derived from the compound of formula (A):C(R1)(R3)═C(R2)—C(═O)—O—(CH2—CH2—O)n—R4  (A)in which the R1, R2 and R3 radicals, which are identical or different, represent, independently of one another, a hydrogen atom or a linear or branched alkyl radical comprising from 1 to 4 carbon atoms, the R4 radical represents a saturated or unsaturated and linear or branched aliphatic radical comprising from 6 to 30 carbon atoms and n represents a number between 1 and 50.
However, when this inverse latex is used to prepare a thickened formulation, its rate of inversion in the aqueous phase, that is to say the time necessary to obtain the maximum development of the viscosity, remains fairly low, which means, for the user, a loss in time which is harmful in the industrial phase. This is because it is well known that the inversion time of inverse latexes increases considerably as a function of the scale of use. Furthermore, the stability over time of the inverse latexes described in FR 2 879 607 is not completely satisfactory. This is because a phenomenon of syneresis is observed with fairly rapid appearance, during storage, of oil at the surface and of a polymer-based residue.